Transient filter



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WM Q Wow-143k 3,026,757 Patented Mar. 27, 1962 3,026,757 TRANSIENT FILTER Robert J. Ziehlke, Janesville, Wis, assignor to Gibbs Manufacturing & Research Corporation, a corporation of Wisconsin Filed Nov. 24, 1958, Ser. No. 775,893 1 Claim. (Cl. 84-119) This invention relates to an electronic musical instrument and more particularly to a novel circuit for an electronic musical instrument, including filter means for reducing the relative amplitude of or eliminating objectionable transient effects.

This application is a continuation-in-part of copending Ziehlke application, Serial No. 745,564, filed June 30, 1958, and assigned to the assignee of this application.

In an electronic musical instrument, as an electronic organ, there are various devices in the signal handling circuitry which may introduce objectionable transient signals into the system, and it is extremely desirable that these transients be eliminated or reduced in amplitude so. that they are not present to an appreciable degree in the output of the instrument. For example, in a preferred form of tone generator, which utilizes a plurality of selectively operable light bulbs or lamps in the generation of the tone representing signals, the thermal or incandescent characteristic of the lamps may give rise to a transient upon energization and deenergization of the lamps. Particularly in an electronic organ, a manually operable swell or volume control is provided; and in many swell control circuits, a transient is introduced corresponding in frequency with the rate of operation of the manual swell control. Furthermore, in many electronic organ systems, means are provided for producing a tremolo effect in which the amplitude of the tone representing signals is varied at a low or sub-audio rate, introducing a transient at the sub-audio rate. These transients have an undesirable effect if reproduced with the tones generated by the instrument and should be eliminated from the tone representing signal before reproduction.

A principal object of the present invention is the provision in an electronic musical instrument of a novel filter means for reducing the amplitude of objectionable transients relative to the amplitude of the tone representing signal.

One feature of the invention is the provision in an electronic musical instrument in which objectionable tran sients are introduced into the signal, at a frequency lower than the lowest frequency of the tone representing signal, wherein a high pass filter is connected to the output of a cathode follower for reducing the amplitude of the transients relative to the amplitude of the tone representing signal. Another feature of the invention is the filter which includes a capacitor connected in series with a winding of a coupling transformer, the capacitor and transformer forming a series resonant circuit at a frequency of the order of the lowest frequency of the tone generating means. A further feature is that the filter has an attenuation characteristic with a slope of the order of twelve decibels per octave at frequencies below the resonant frequency thereof.

Still another feature is that the transient filter is used in an electronic musical instrument having tone generating means including a plurality of selectively operable lamps, the light from the lamps being converted into an electrical tone representing signal, and the thermal characteristics of the lamps introducing objectionable transients into the tone representing signal. Yet a further feature is that the transient filter is used in an electronic musical instrument having circuitry including a swell control circuit having a manually operable control, actuathe tone representing signal, and a selectively operable tremolo system including a variable impedance device controlled by a sub-audio oscillator and introducing an objectionable transient at the sub-audio rate.

Further features and advantages of the invention will readily be apparent from the following specification and from the drawings, in which:

FIGURE 1 is a block diagram of an electronic musical instrument embodying the invention;

FIGURES 2 and 2A are a schematic circuit diagram thereof;

FIGURE 3 is a curve illustrating the transient characteristic of a lamp; and

FIGURE 4 is a curve illustrating a characteristic of the transient filter.

In FIGURE 1 of the drawings, a block diagram of an electronic musical instrument, here an organ, is illustrated, having a plurality of tone generators 10, the output of which is connected with a preamplifier 11. The amplitude of the output signal from preamplifier 11 is controlled by means of swell control 12 with Which the organist may control the ultimate volume of the generated tones. The signals from the preamplifier 11 are coupled to a further amplifier 13, which is provided with a tremolo control 14 which may be rendered operative by the tremolo control switch 14a. The purpose of the tremolo is to impart to the signal a tremolant quality, i.e., a slight variation in amplitude, preferably above and below the normal amplitude of the signal, at a low or sub-audio rate. As will appear more fully below, certain characteristics of the various elements of the system, including tone generators and the swell and tremolo control circuitry may introduce into the tone representing signal a low frequency transient voltage which will have an undesirable effect on the fidelity of the instrument. In accordance with the present invention, a filter 15 is provided in the signal handling channel for eliminating or reducing the amplitude of the objectionable voltage with respect to the desired tone representing signal. The output of the filter is coupled with power amplifier 16, and to a speaker system 17.

Turning now to FIGURES 2 and 2A, a schematic circuit diagram of a musical instrument embodying the invention will be described in some detail. The circuits of FIGURES 2 and 2A are connected by terminals A-A and B -B. During the course of this description, tube types and component values for many of the elements of the system will be given. It will be understood that this specific information is given for the purpose of making a complete disclosure of an operative embodiment of the invention, and the values given are not to be considered critical, as many changes will be apparent to those skilled in the art.

Lamps 21 are energized from a suitable source of electrical energy 22, which may, for example, have an output voltage of ten volts DC. The connection of lamps 21 to the power source 22 is effected by key switches 23, which may be associated with the keys of a standard instrument keyboard, through a stop selector means 24 for choosing the lamps associated with the desired tone patterns. The photocells 20 for each of the tone generators 10 (of which there may be twelve, one for each note in an octave) are connected in parallel and through a current limiting resistor 25, l megohm, with a suitable B+ supply source. The tone representing signals are connected through capacitor 26, 0.01 ,uf. (microfarad), to the control grid of a cascade connected dual triode amplifier '27, a l2AT7.

The characteristics of the filament of lamps 21 are such that the lamps do not shift instantaneously to a full incandescent condition when the key switch is closed, nor do they drop instantaneously to a dark condition when the key switch is opened. As shown in FIGURE 3, a finite period t is required between the'instant the key is closed and the time at Which the average D.C. level of the tone is achieved. Similarly, a finite period I is required between the time the key is opened and the point where the output from the photocell due to light from the lamp become insignificant. During these transition periods, t and t transients are generated in the system which are a result of the shift in D.C. level across photocells 20. The frequencies of the transients is dependent principally upon the thermal or incandescent characteristics of the filaments of lamps 21, but primarily they are composed of low orsub-audio frequencies.

The output of cascade connected amplifier 27 is connected through a series resistor 28 and coupling capacitor 29 with the control grid of triode amplifier 30, onehalf ofa 1-2AT7.

Shunted across the signal channel between amplifiers 27 and 30 is a swell control circuit including a variable impedance device 31, one-half of a l2-AT7. The conduction through device 31 is controlled by the swell pedal 12,

as will appear, varying the apparent impedance shunting the. signal channel, and thus controlling the relative amplitude of the tone representing signals transmitted through the channel. i

In a preferred embodiment of the system, swell control 12 actuates a mask 32' interposed between lamp 33 and a photocell 34 which is connected to resistor 35 inthe circuit of the control grid of variable impedance device 31. With the swell control pedal fully raised, i.e.,

minimum volume, the full light from lamp 33 shines on photocell 34 and it conducts heavily, applying a positive potential through resistor 36 to the control grid of tube 31.. In turn, tube 31 conducts heavily and appears as a low impedance to the signal channel. As the control pedal is depressed, mask 32 progressively cuts off the light from lamp 33 shining on photocell 34, reducing the positive potential applied to the control grid of tube 31 and increasing its apparent impedance. When the swell pedal is fully depressed, the current through tube 31 is at a minimum and its impedance is a maximum, 2. condition of maximum signal volume.

As the swell pedal is operated and-the current through tube 31' changes, the D.C. potential of the anode also changes at a rate dependent upon the speed of movement of the control pedal. A transient signal thus produced is coupled through capacitor 37, 0.1 ,uf., to the signal channel; In an elfort to reduce the relative amplitude of the transient signal, a feedback capacitor 38, 0.004 f, is connected between'the anode and cathode tube 31, providing a feedback signal. However, this feedback circuit cannot completely eliminate the transient effect, and a certain portion of the undesired transient is added to the tone representing signal in the signal channel.

Returning now to a consideration of the primary signal handling channel, the output of amplifier 30 is coupled through capacitor 40 to a manually operable level control potentiometer 41. The movable tap for potentiometer 41 is connected through series dropping resistors 42 and 43, to the control grid 44aof amplifier 44, a 12AX7. Shunted acrossthe signal channel between series resistors 42 and 43, is a selectively operable tremolo circuit including a triode 45, one-half of a 12AT7, whose anode is connected with the signal channel through capacitor 46, 0.025 nf. The control grid circuit of tube 45 is selectively connectable with a sub-audio oscillator 47, through tremolo control switch 14a. With switch 14a in the position shown in the drawing, connecting the sub-audio oscillator with the control grid. circuit of tube 45, to shunt impedance of the stage is varied at the frequency of the oscillator, which may be, for example, four or five cycles per second. The tremolo circuit operates in a manner similar to the swell control circuit, varying. the

shunt impedance presented to the signal channel, and thus varying the relative amplitude of the tone representing signal. Furthermore, as in the swell control circuit, the operation of the tremolo introduces transient signals into the signal handling channel, here basically at the frequency of the sub-audio oscillator. Again, a feedback circuit including capacitor 48, 0.0l' f., is connected between the anode and the control grid of tube 45 in an effort to reduce the transient effect, but it is not completely eliminated.

The amplifier 44 includes an anode 44b connected to an output terminal 60 and a cathode 440 connected to a terminal 61 by a resistor 62. The terminal 61 is connected to the grounded negative supply terminal. The terminal 60 is connected to a positive supply terminal 64 by resistors 65 and 66. i

A bass boost circuit 50 is shunted across the input of amplifier 44 and the output of the amplifier is connected through capacitor 51 with the control grid 52a of cathode follower tube 52,;one-half of a 12AT7. The cathode 52b of the cathode follower tube is connected to ground potential by a load resistance which is divided into two portions, agrid bias resistor 53, 1000 ohms, and an output load resistor 54, 33,000 ohms, with the output potential being derived across resistor 54. The control grid is returned through resistor 55 to the juncture of the resistors 53 and 54 providing some degenerative feedback. The anode 520 of the cathode follower tube is connected to the positive supply terminal 64 by the resistor 66. The output of the cathode follower is coupled through capacitor 56 to the primary winding-57a of coupling transformer 57. The secondary winding 57b of coupling transformer 57 has a grounded center tap 57b and providesv two signals, out of phase with each other, for push pull driver amplifiers 58a and 58b, the two halves of a twin triode, l2AU7. The output of drivers 58a and 58b is coupled to power amplifier 1.6 and from there to speaker 17.

As is apparent from the foregoing discussion, =the varigenerated by the tremolo circuit are primarily the sub audio frequency of the tremolo oscillator 47.

In accordance with the present invention, a filter network is incorporated in the signal channel to reduce'the relative amplitude of the undesirable transient frequencies. In the illustrated embodiment of the invention, capacitor 56 forms a series resonant circuit in the input of coupling transformer 57. The resonant frequency of the filter circuit is preferably of the order of or slightly higher than the frequency of the lowest generated tone. In a specific embodiment of the system where lowest generated tone has a frequency of 16 c.p.s., the resonant frequency of the filter is 32. c.p.s. Forthis resonant frequency, capacitor 56 has a value of 1.25 ,uf, and the input of transformer 57 an apparent inductance of the order of 20 henries. The transfer characteristic of filter 15 is illustrated in FIGURE 4, where the ratio of the output vol" age to the input voltage of the filter is plottedas a function of frequency. It will be noted that the characteristic has a slight peak at the resonant frequency (here 32 c.p.s.), and then drops oif rather rapidly. The slope of the attentuation characteristic of the filter below the resonant frequency f is preferably of the order of twelve decibels per octave, so that the low transient frequencies are highly attentuated. For example, the tremolo frequency from sub-audio oscillator 47 is about three octaves below the resonant frequency of the filter, and is attenuated 36 decibles below the level of the tone representing signals. I

While I have shown and'described certainembodiment-s of my invention, it is to be understood that it is capable of many modifications. Changes therefore, in the construction and arrangement may be made without departing from the spirit and scope of the invention as disclosed in the appended claim.

I claim:

In a musical instrument of the type in which first and second tone signal terminals have applied thereto musical tone signals extending over a range of several octaves together with undesired transient noise substantially lower in frequency than the lowest pitch tone signal and in which a positive and a negative supply potential terminals are connected respectively to the first and second tone signal terminals, in combination with the terminals, a cathode follower including an electron discharge tube having an anode connected 'to the positive supply potential terminal, a grid connected to the first tone signal terminal, a cathode, and a load resistor connecting the cathode to the second tone signal terminal; a voltage gain coupling transformer having a primary winding and a secondary winding; and a capacitor connected in series with the primary winding and forming with the primary Winding a series resonant circuit having a resonant frequency approximately equal to the pitch of the lowest pitch musical tone signal, the series connected capacitor and primary winding being connected across the load resistor to induce voltage amplified tone signals in the secondary winding and to substantially attenuate the low frequency transient noise.

References Cited in the file of this patent UNITED STATES PATENTS UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,026,757 March 27, 1962 Robert J. Ziehlke It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 2, line 56 before "Lamps 21" insert thefollowing:

A preferred form of tone generator 10 is indicated schematically in Figures 2 and 2A, each tone generator including a photocell 20 having a plurality of selectively operable light sources or lamps 21 associated therewith. The light from lamps 21 is utilized to form tone signals in a well known manner.

column 3, line 71, for "to" read the Signed and sealed this 10th day of July 1962.

(SEAL) Attest:

ERNEST W. SWIDER DAVID L. LADD Attesting Officer Commissioner of Patents 

